Pneumatic servo assembly for an electro-pneumatic converter

ABSTRACT

A pneumatic servo assembly including a back pressure nozzle and an electric motor operating a flat spiral cam variably restricting the nozzle. A nozzle clamp exerts a drag on the cam so that a constant pneumatic output is maintained if an electrical failure occurs.

TECHNICAL FIELD

This application is a continuation of application Ser. No 637,948 filedAug. 6, 1984 which is a division of application Ser. No. 469,200 filed2-24-83 both abandoned.

The present invention relates to control systems for electro-pneumaticconverters in general and particularly to pneumatic servo assemblies forsuch control systems utilizing a variable restriction cam andbackpressure nozzle feeding an output bellows which also provides afeedback signal to the control system.

BACKGROUND ART

Control systems for electro-pneumatic converters are known. Usually a 4to 20 milliamp electrical signal is used to actuate a solenoid-likemotor. The 4 to 20 milliamp electrical signal causes a proportionatedisplacement in the spring-loaded core of the solenoidlike motor whichdisplacement is used to control the restriction of an associatedpneumatic valve producing a pressure change proportional to the motionof the core. An example of such a device may be found in U.S. Pat. No.3,334,642 issued Aug. 8, 1967 to P. G. Borthwick.

There are certain disadvantages to the pneumatic servo assemblies ofsuch electro-pneumatic control systems. Firstly, they are unable to holdpositions on loss of power. Should power be removed from the coil, thecore moves back to a position where it is in equilibrium with itsassociated spring. This causes the pneumatic output signal to go offscale, resulting in the movement of control devices actuated by theelectropneumatic system to either the fully-opened or fullyclosedpositions which may be catastrophic under certain circumstances.Secondly, such pneumatic servo assemblies are vibration sensitive. Sincethe cores are suspended from springs which act as range and zerolimiters, vibration of the core causes a variation in the pneumaticoutput signal. Also, there usually is no feedback signal of thepneumatic output signal to the input of the control systems.

SUMMARY OF THE INVENTION

The present invention overcomes these problems of knownelectro-pneumatic control systems as well as others by providing apneumatic servo assembly for such systems which is dependent upon anelectrically-driven D.C. motor to provide a variable restriction to apneumatic nozzle, thereby providing a fail-safe device which willmaintain the last electrical signal to the pneumatic assembly upon aloss of electrical power since the motor will stop in its last drivenposition.

The pneumatic servo assembly of the present invention utilizes a D.C.motor-driven cam member to provide a variable restriction to a pneumaticbackpressure nozzle thus allowing the nozzle to supply a springloadedbellows assembly which produces a 3 to 15 psi pneumatic output signalalso providing a feedback signal to the electrical input signal.

The feedback signal is used to produce an error signal between a setpoint signal determined by the 4 20 milliamp electrical input and thefeedback signal of the pneumatic output as sensed by a pressuretransducer changing this pneumatic feedback signal to anelectrically-equivalent signal.

Thus, one object of the present invention is to provide a pneumaticassembly for an electro-pneumatic control system which will maintain thelast pneumatic output upon a loss of electric power.

Another object of the present invention is to provide a pneumaticassembly for an electro-pneumatic control system which is insensitive tovibration of the pneumatic assembly.

Yet another object of the present invention is to provide a pneumaticassembly for an electro-pneumatic control system which provides afeedback signal to the electronic part of the control system producingan error signal driving the restriction of the pneumatic assembly.

These and other objects of the present invention will be more clearlyunderstood from a review of the following detailed description of theinvention when considered with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of the control system of thepresent invention.

FIG. 1a is an expanded view of the motor-driven cam and backpressurenozzle of the mechanical servo assembly of FIG. 1.

FIG. 2 is a functional block diagram of the pneumatic servo assembly ofthe FIG. 1 control system.

FIG. 3 is a detailed side view of the bellows and spring assembly ofFIG. 2.

FIG. 4 is a detailed end view of the FIG. 3 bellows and spring assembly.

FIG. 5 is a detailed end view of the cam assembly 23 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are made for purposesof illustrating the preferred embodiment of the present invention andare not intended to limit the invention thereto FIGS. 1 and 1a show anelectro-pneumatic control system 10 wherein a D.C. motor 12 iscontrolled by an electronically-controlled motor servo circuit 14 whichis powered by a power supply 16 operated from a 4 to 20 milliamp inputcontrol signal connected to the power supply 16 along line 18. The D.C.motor 12 is mechanically constrained to a pneumatic servo assembly 20which has a backpressure nozzle 21 variably restricted by a cam assembly23 connected to and driven by the D.C. motor 12 to thus provide avariable backpressure output along output line 22 of the pneumatic servoassembly 20 normally in the 3 to 15 psi output range. This 3 to 15 psioutput is linear and corresponds to the linear 4 to 20 milliampelectrical input provided along input line 18. This same 3 to 15 psioutput is also sent along line 24 to a pressure transducer 26 whichprovides a feedback signal used in determining control of the D.C. motor12 as will be described more fully later.

To allow the D.C. motor 12 to be operated bidirectionally without theneed for dual polarity voltages, the power supply 16 establishes dualvoltages V+ and V_(ref) along power lines 28 and 30 respectively. The V+voltage is in the range of 6.4 volts nominal and powers the motor servocircuit 14 as well as a desired position amplifier 38 and an amplifiercircuit 46 along input lines 31 and 33 respectively. The V_(ref) portionof power supply 16 is transmitted along line 30 to bias up the motorservo circuitry 14 and power the pressure transducer 26 along line 32.

To establish the set point from the 4 to 20 milliamp input signal towhich the appropriate 3 to 15 psi output will have to be supplied fromoutput line 22, the particular electrical input signal is sent alongline 34 to a 10 ohm precision resistor located between the circuitcommon at line 36 and the input to a position amplifier 38. Theprecision 10 ohm resistor senses the particular current level andestablishes a voltage drop across itself with that voltage dropproviding the counterpart voltage input to the position amplifier 38.The position amplifier 38 raises the input signal level to apredetermined level and sends this along line 40 as a set point signalto a difference amplifier 42 compatible with the level of the feedbacksignal also provided to the difference amplifier 42. The differenceamplifier 42 is the first stage of the motor servo circuit 14. Thesecond input to the difference amplifier 42 is the feedback signalprovided along line 44 from amplifier circuit 46 which scales and zeroesthe pressure signal provided by pressure transducer 26 which acts as thepneumatic-to-electric converter for the 3 to 15 psi output signalestablished at output line 22.

The difference amplifier 42 senses any deviation of the feedback signalfrom line 44 to the established set point signal 40 and establishes anerror signal along line 48 which is an amplified difference signal solong as such difference between set point and feedback is maintained.This amplified error signal is inputed into a proportional and integralcontroller 50 where it is integrated and scaled up or down with respectto V_(ref).

Thus bi-directional rotation of the motor 12 is achieved by the voltageoutput of the proportional plus integral controller 50 rising or fallingbelow the voltage reference V_(ref). When the output signal is abovevoltage reference V_(ref)., the current through the motor 12 will drivethe motor 12 in a first rotational direction. When the voltage output ofthe proportional plus integral controller 50 is equal to V_(ref)., nocurrent flows through the motor and the motor is stationary. If theoutput voltage drops below V_(ref)., the rotation of the motor 12 willreverse to a second rotational direction due to the voltage levelapplied to it crossing the V_(ref). point.

Turning back now with particular reference to FIG. 1, the amplifiercircuit 46 has both a zero adjustment 60 and a span adjustment 62. Thezero and span adjustment allows the feedback signal to be adjusted torespond over a variety of ranges. The predominant pressure range andpressure starting or zero point that the feedback will be adjusted foris the 3 to 15 psi signal which is the standard for pneumaticinstrumentation as 4 to 20 milliamp is the standard for electricalinstrumentation. Other ranges are also available and may be set,including any 50 percent split range desired (i.e., 0 percent is 3 psi,100 percent is 9 psi).

Should an electrical failure occur in the system 10, the D.C. motor 12would stop with the cam 23 remaining in its last position to provide thesame backpressure restriction from the nozzle 21 to the pneumatic servoassembly 20 and the last conforming pressure output signal would bemaintained along output line 22 by the pneumatic servo assembly 20.

Referring now to FIG. 2, the pneumatic servo mechanism 20 is seen toinclude a regulator 64 which is connected to an air supply ofunregulated high pressure air and acts to reduce the air supply pressureto a constant clean, low pressure of 22+2 psi. The filtered andregulated air from regulator 64 is piped to the backpressure nozzle 21by way of an orifice 66. As is known to those in the pneumatic controlarts area, the size of the orifice 66, in conjunction with the openingof the backpressure nozzle 21, act to determine the air consumption aswell as response time of the pneumatic servo mechanism 20.

As was previously described, the motor servo assembly 14 causes the D.C.motor 12 to be rotated in either a clockwise or counterclockwisedirection which direction is dictated by a comparison of the set pointand the feedback signals inputed to the control circuit 14 which acts tothus control the D.C. motor 12. The rotation of the motor 12 causes thecam assembly 23, which may be best seen at FIG. 5, to rotate withrespect to the backpressure nozzle 21 causing a relative blockage oropening of the backpressure nozzle 21.

With particular reference to FIG. 5, it will be seen that the camassembly 23, shown as the typical 3 to 15 psi output cam assembly 23, isformed as a spiralgenerated plane 68 having a notched portion 70 with ahub 72 located in the center of the spiral plane section 68. The spiralis formed to produce a linear function pressure output from output line22 from the backpressure nozzle 21. By way of example, when the nozzle,which will always seek the edge of the plane 68 aligned therewith, ispositioned with point A a 3 psi output will be produced. Similarly, atpoint B, a 15 psi output will be produced. Angularly linear outputs willbe produced between points A and B. Thus, the height of the notch 70 isthe range of the output signal. The hub 72 is used to mount the camassembly 23 to the shaft of the D.C. motor. Turning next to FIGS. 2through 4, it will be seen that restricting the backpressure nozzle 21causes an increase in backpressure which is piped to the bellows springassembly 74 through line 76. There is a directly-proportionalrelationship between the pressure in the bellows spring assembly 74 andthe height to which it will expand. This is determined by theconstruction of the bellows 78 as well as the spring 80 which is mountedin parallel with the bellows 78. The spring 80 acts to limit the motionof the bellows 78, thereby limiting the output range of the pneumaticoutput signal along line 22 to a desired range which is determinable byadjusting the spring pressure of the spring 80 by either extending orloosening the spring 80 and setting it in that particular position byway of adjusting nuts 82. Thus, nuts 82 may be used to provide fineadjustment to the particular output pressure range desired. Should adifferent pressure range be desired, such as a 3 to 27 psi, a differentspring 80 having a different spring coefficient may be replaced.

With particular reference to FIGS. 3 and 4, it will be seen that thebackpressure nozzle 21 is rigidlymounted to a bracket assembly 84 towhich the bellows 78 and the spring 80 are also mounted. The bracketassembly 84 is then mounted to a stationary frame member 86 through ahinge 88 to thus allow rotational motion of the backpressure nozzle 21and the bellows 78 and spring 80 around the pivot point 88.

This mounting of the backpressure nozzle 21, bellows 78, and spring 80as a single unit makes the pneumatic servo assembly impervious tovibration induced errors by allowing the entire assembly to move as asingle unit in response to any vibration induced by external sourcesinto the pneumatic servo assembly 20.

In operation, it will be seen that as the motor 12 and cam assembly 23rotate to variably restrict the backpressure nozzle 21, the bellows 78will expand causing a pivoting of the previously-mentioned assemblyaround pivot point 88 until the backpressure nozzle 21 reaches aposition along the edge of the cam plane surface 6 producing abackpressure feedback signal which will balance the set point signal andstop the rotation of the motor 12 and cam assembly 23. This will resultin an output pressure signal along line 22 which is proportional to theelectrical input signal which originally had caused the D.C. motor 12 torotate.

As may be seen, there is no mechanical contact between the backpressurenozzle 21 and the cam assembly 23. Therefore, the D.C. motor 12 needonly overcome its own internal friction to rotate along with a smallamount of drag on the cam assembly 23 which may be caused by the nozzleclamp 86. The nozzle clamp 86 rides on the cam assembly 23 loading itaway from the motor 12 to thus take up any end play in the shaft of themotor 12. This force is minimal and there are no forces developed whichwill turn the motor 12 off. If the motor 12 and the cam assembly 23 donot turn clearly the backpressure output along line 22 will not change.

Certain modifications and improvements will occur to those skilled inthe art upon reading the foregoing Specification. It will be understoodthat all such improvements and modifications are deleted herein for thesake of conciseness and readability but are properly intended to bewithin the scope of the following claims.

I claim:
 1. A pneumatic servo assembly for an electropneumatic closedloop control system providing a pneumatic output signal in response to acorresponding input signal comprising:variable restriction means forproducing different pneumatic output signals from the pneumatic servoassembly; motor means for moving said variable restriction means to varythe pneumatic output from the pneumatic servo assembly, said motor meanshaving a cam assembly connected thereto for variably restricting thepneumatic servo assembly depending on the position of the cam; abackpressure nozzle mounted proximately to said cam assembly to bevariable restricted by said cam assembly; means for actuating said motormeans in response to a control signal; means for establishing a setpoint signal in response to the electrical input signal indicative of adesired pneumatic output signal; means for establishing a feedbacksignal indicative of the pneumatic output signal; combining means forcomprising the setpoint signal with the feedback signal to establish acontrol signal to said actuating means; and a nozzle clamp for exertingdrag on the cam assembly to maintain a constant pneumatic output signalfrom said pneumatic servo assembly corresponding to the last controlsignal to said pneumatic servo assembly in response to an electricalfailure.
 2. A pneumatic servo assembly as set forth in claim 1 whereinsaid feedback establishing means includes a pressure transducerconnected to the pneumatic servo assembly to measure the pneumaticoutput therefrom and establish an electrical signal representativethereof.
 3. A pneumatic servo assembly as set forth in claim 2 whereinsaid feedback establishing means further includes an amplifier circuithaving a span and zero adjustment to allow the pneumatic output fromsaid pneumatic servo assembly to be scaled to a desired range of outputpressures from a desired starting pressure.
 4. A pneumatic servoassembly as set forth in claim 1 wherein said motor means is a D.C.motor.
 5. A pneumatic servo assembly as set forth in claim 4 whereinsaid cam assembly is a flat, substantially spiral-generated plane havinga generally center mounted hub for mounting the cam assembly to saidmotor means.
 6. A pneumatic servo assembly as set forth in claim 23wherein said backpressure nozzle is connected to a bellows assembly topressurize said bellows assembly in response to the pressure from saidbackpressure nozzle.
 7. A pneumatic servo assembly as set forth in claim6 wherein said bellows assembly has an output connected to said feedbacksignal establishing means.
 8. A pneumatic servo assembly as set forth inclaim 7 wherein said bellows assembly includes an adjustable springconnected in parallel with the bellows to adjust the output from thebellows assembly.
 9. A pneumatic servo assembly as set forth in claim 8wherein said backpressure nozzle is rigidly attached to said bellowsassembly to move simultaneously with said bellows assembly.
 10. Apneumatic servo assembly as set forth in claim 9 wherein saidbackpressure nozzle and bellows assembly is attached as a unit to asingle pivot point allowing the rotation of both as a unit around saidpivot point in response to bellows expansion or contraction.
 11. Apneumatic servo assembly as set forth in claim 1 wherein said nozzleclamp is formed as a single member having one end mounted to saidbackpressure nozzle with the opposite end in contact with said camassembly on the side opposite said backpressure nozzle thereby exertingdrag on said cam assembly.